79
80 5. 1. Introduction
Versatile TADF molecular systems, including phthalonitrile,[1] triazine,[2]
benzophenone,[3] diphenylsulfone,[4] and spirofluorene derivatives[5] have been developed for the last couple of years, and high int and external quantum efficiency (ext) have been achieved for OLEDs using these efficient TADF emitters, which are approaching the best performance of phosphorescent OLEDs. Among the key chromaticity components (i.e., red, green, and blue emitters), it is very challenging to explore highly efficient and stable blue TADF molecules because of their intrinsic wide energy gap and high T1 excited energy levels. So far there have been only a few reports on blue TADF emitters exhibiting both the high electroluminescence (EL) efficiency and color purity.[4a,6]
Boron-containing luminescent π-conjugated molecules have been investigated intensively as optoelectronic functional materials,[7] as unique electronic interactions between the vacant 2p orbital of boron and the π* orbital can lead to produce new electron-accepting materials. Kawashima and coworkers reported that functionalized phenazaborins bearing boron and nitrogen atoms on the central π-conjugated framework, exhibit excellent photoluminescent (PL) properties with the PL quantum yields (PL) of up to 100%.[7a] Nevertheless, phenazaborin derivatives have not yet been applied to emitters in OLEDs, and their EL performance has not been clarified up to now. In view of this, utilizing a phenazaborin acceptor unit in TADF molecular design will provide blue TADF emitters with high PL and EL efficiencies.
In this chapter, a new high-efficiency blue TADF luminophore, MFAc-AzB (Figure 5-1) based on angular-linked phenazaborin acceptor and spiroacridan donor units, was designed and synthesized, that accomplishes a remarkably high ext of 18.2% for blue TADF-based OLEDs with a color coordinate of (0.15, 0.23). TADF emission originates from singlet intramolecular charge transfer (ICT) excited state,[1-5] so the emission wavelength should depend on electronic donor and acceptor strength. Aiming at TADF emission at a short wavelength, MFAc-AzB is designed to weaken the ICT effect by combining a relatively weak phenazaborin acceptor with a spiroacridan donor unit. The phenyl-substituted amino moiety in phenazaborin unit can weaken the electron-withdrawing property of a boron atom owing to participation of a lone pair of nitrogen in π-system, and hence lead to blue shift of ICT emission.
81
Figure 5-1. (a) Molecular structure and (b) HOMO (−4.66 eV) and LUMO (−1.51 eV) distributions of MFAc-AzB calculated at the B3LYP/6-31G(d) level. The calculated S1 and T1
energies are 2.62 and 2.59 eV, respectively.
5. 2. Molecular Geometric and Electronic Structures
Time-dependent density functional theory (TD-DFT) calculations were performed at the B3LYP/6-31G(d) level for understanding the electronic and geometrical structures of MFAc-AzB. As shown in Figure 5-1b, the calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are well separated and distributed on spiroacridan donor and phenazaborin acceptor units, respectively, on account of a large dihedral angle of 89.1° between these units. As a result, this clear spatial separation of the frontier orbitals leads to a very small energy gap between the S1 and T1 states (ΔEST) of 0.03 eV, which is required to promote TADF, because the RISC rate constant (kRISC) is inversely proportional to the exponential ΔEST, following a Boltzmann relationship: kRISC ∝ exp(−ΔEST /kBT).
Figure 5-2. (a) Absorption and PL spectra of MFAc-AzB in toluene solution and 20 wt%
doped film in PPF at room temperature. (b) Photographs of luminescence recorded under UV irradiation at 365 nm.
HOMO LUMO
FMAc-AzB
(a) (b)
82
Figure 5-3. (a) Streak image and time-dependent PL emission spectra showing the prompt (black) and delayed (red) components, and (b) transient PL decay curve of a 20 wt%-MFAc-AzB:PPF doped film at 300 K. (c) Temperature dependence of transient PL decay ranging from 5 to 300 K. (d) Postulated PL decay processes for MFAc-AzB; krS and knrS are the radiative and non-radiative decay rate constants of the S1 state, kISC and kRISC are the ISC (S1→T1) and RISC (T1→S1) rate constants, respectively, and knrT is the non-radiative rate constant of the T1 state.
5. 3. Photophyscial Properties
Figure 5-2 presents the UV–vis absorption and PL spectra of MFAc-AzB. The lowest-energy absorption was observed at around 390 nm in toluene solution, which is assigned to the ICT transition mainly associated with electron transfer from the spiroacridan to phenazaborin moieties. MFAc-AzB emitted blue PL light with an emission peak (λPL) at 464 nm, and the absolute PL of MFAc-AzB in toluene increased from 13% to 26% after N2 bubbling. The PL spectrum of a 20 wt%-MFAc-AzB:PPF (PPF = 2,8-bis(diphenylphosphine oxide)dibenzofuran)[8] doped film showed almost the same blue PL emission (λPL = 467 nm) solely from the MFAc-AzB emitter without the emission from the PPF host, suggesting that energy transfer from the host to emitter is efficient in the doped film. Remarkably, the absolute
PL of an MFAc-AzB neat film and MFAc-AzB:PPF doped film reached to 53% and 99%
under N2 at room temperature, respectively, which are much higher than that in dilute solution,
83
because of reduced torsional and vibrational non-radiative relaxations in the solid states. Such remarkable enhancement of the PL properties in the neat film, compared to its dilute solution, reveals the aggregation-induced emission (AIE)[9] characteristics of MFAc-AzB. In the doped film, further suppression of concentration quenching effect seems to enhance the PL to almost 100%.
To gain further insight into the photophysical properties, the transient PL decay and its temperature dependence were analyzed for a 20 wt%-MFAc-AzB:PPF doped film by using a streak camera (Figure 5-3). At room temperature (300 K), the transient PL decay can be evidently classified into two components (Figures 5-3a, b). The first one corresponds to nanosecond-scale prompt fluorescence (lifetime τp = 19 ns) from the S1 to the ground state (S0).
The second one is assigned to microsecond-scale TADF (τd = 91 μs). The time-dependent PL spectra for the prompt and delayed components are coincident each other. While the TADF component is almost negligible below 50 K, the intensity of TADF obviously increased with increasing temperature to 300 K (Figure 5-3c). Thus, it can be estimated that the overall PL
of 99% in the doped film is composed of 35% for the prompt component (p) and 64% for the delayed component (d). The p and d efficiencies were derived from the emission intensity proportions of the prompt and delayed components in the transient PL signals. The experimental ΔEST of MFAc-AzB in the doped film was estimated to be 0.24 eV from the S1 and T1 energy levels of 2.84 and 2.60 eV determined by the measured fluorescence (300 K) and phosphorescence (5 K) spectra (Figure 5-4 and Table 5-2). From the experimentally obtained PL efficiency and lifetime data, the radiative decay rate constant for S1→S0 (krS), the intersystem crossing rate constant for S1→T1 (kISC), and the RISC rate constant for T1→S1 (kRISC) of MFAc-AzB in the doped film are calculated to be 6.4 × 106, 1.2 × 107, and 2.0 × 104 s−1, respectively (Figure 5-3d and Table 5-3).
Table 5-2. Photophysical data of MFAc-AzB.
Compound λasb [nm]
sola)
λPL [nm] ΦPL[%]c) τp [ns]d) / τd [μs]d)
HOMO [eV]e)
LUMO [eV]f)
ES / ET
[eV]g)
ΔEST
[eV]h) sola) / filmb) sola) / filmb)
MFAc-AzB 307, 373, 387 464 / 464 26 / 99 19 / 91 −5.85 −2.90 2.84 / 2.60 0.24 a)Measured in oxygen-free toluene solution at room temperature; b) 20 wt%-doped thin film in a host matrix (host = PPF); c)Absolute PL quantum yield evaluated using an integrating sphere under a nitrogen atmosphere; d)PL lifetimes of prompt (p) and delayed (d) decay components for the 20 wt%-doped film measured using a streak camera at 300 K; e)Determined by photoelectron yield spectroscopy in pure neat films; f)Deduced from the HOMO and optical energy gap (Eg); g)Singlet (ES) and triplet (ET) energies estimated from onset wavelengths of the emission spectra at 300 and 5 K in the doped films, respectively; h)ΔEST = ES−ET.
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Figure 5-4. PL spectra for prompt fluorescence in the range of 0–300 ns at 300 K (black) and phosphorescence in the range of 1–10 ms at 5 K (red) for 20 wt%-MFAc-AzB-doped film in a PPF host matrix. The lowest excited singlet (S1) and triplet energy (T1) levels of MFAc-AzB were determined from the high energy onsets of the fluorescence and phosphorescence, respectively.
Table 5-3. Rate constants and quantum efficiencies for decay processes of MFAc-AzB in the 20 wt%-doped film.
Compound krS [s−1]
kISC [s−1]
kRISC [s−1]
p [%]
d [%]
ISC [%]
RISC
[%]
MFAc-AzB 6.8×106 5.6×106 2.3×106 17 62 83 75
a)Abbreviations: krS, radiative rate constant (S1→S0); kd, delayed-radiative rate constant (S1→T1→S1→S0); kISC, intersystem-crossing (ISC) rate constant (S1→T1); kRISC, reverse ISC rate constant (T1→S1); p, quantum efficiency for prompt fluorescence component; d, quantum efficiency for delayed fluorescence component; ISC, ISC quantum efficiency; RISC, RISC quantum efficiency.
5. 4. Electroluminescence Performance
Next, the performance of MFAc-AzB as a blue TADF emitter was studied by fabricating a multilayer OLED with the following device configuration (Figure 5-5a): ITO (100 nm)/HAT-CN (10 nm)/αc-NPD (40 nm)/mCP (10 nm)/20 wt%-MFAc-AzB:PPF (20 nm)/PPF (10 nm)/TPBi (30 nm)/Liq (0.8 nm)/Al (80 nm). In this device, HAT-CN (2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene) and α-NPD (4,4'-bis[N-(1-naphthyl)-N-phenylamino]-1,1'- biphenyl) were used as a hole-injection layer and hole-transporting layer; whereas TPBi (1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene) and Liq (8-hydroxyquinoline lithium) were employed as an electron-transporting layer and electron-injection material, respectively. In addition, thin layers of mCP (1,5-bis(9-carbazolyl)benzene) and PPF with high T1 energies (T1
= 2.9 eV[10] and 3.1 eV,[8] respectively) were inserted to suppress triplet exciton quenching at the neighboring interfaces and to confine the excitons in the MFAc-AzB:PPF emitting layer.
350 400 450 500 550 600
0.0 0.2 0.4 0.6 0.8
1.0 Flu.
Phos.
PL intensity (a. u.)
Wavelength (nm)
85
The current density–voltage–luminance (J–V–L) characteristics, ext plots, and EL spectrum of the MFAc-AzB-based TADF OLED are depicted in Figure 5-5. The device exhibited bright blue EL emission with the peak at 473 nm, and the EL spectrum was similar to the corresponding PL spectra (Figure 5-2a). Moreover, the MFAc-AzB-based TADF OLED achieved a high ext of 18.2%, current efficiency (c) of 32.6 cd A−1, and power efficiency (p) of 25.6 lm W−1 at low current densities. Commission Internationale de l'Éclairage (CIE) color coordinates of the device were (0.15, 0.23). In general for TADF OLEDs, the T1 excitons are directly formed by carrier recombination and then converted to the emissive S1 excited state through efficient RISC. Therefore, the theoretical maxima of int and ext can be estimated by equations 1-18 and 1-19. Based on the foregoing equations, int of the MFAc-AzB-based device can be estimated to be 98–99%, which is nearly four times higher than the 25% limit of int of conventional fluorescent OLEDs. This study has thus achieved a highly efficient blue TADF OLED with ext over 18%, which is the highest level among blue-emitting TADF OLEDs ever reported.[4a,6]
Figure 5-5. (a) Energy-level diagram, (b) current density–voltage–luminance (J–V–L) plots, and (c) external EL quantum efficiency as a function of current density for a blue TADF OLED based on MFAc-AzB as an emitter. The insets in (c) show and an EL spectrum and a photograph of blue EL emission from the device taken at 10 mA cm−2.
External EL quantum efficiency (%)
Current density (mA cm–2) Voltage (V)
Current density (mA cm–2) Luminance (cd m–2)
86
Table 5-4. EL performance of the TADF-based OLED.a)
TADF
emitter Host λEL
[nm]
Von
[V]
Lmax [cd m−2]
ηext
[%]
ηc [cd A−1]
ηp
[lm W−1] CIE (x, y) MFAc-AzB PPF 473 4.0 1,900 18.2 32.6 25.6 (0.15, 0.23) a)Abbreviations: λEL = EL emission maximum, Von = turn-on voltage at 1 cd m−2, Lmax = maximum luminance, ηext = maximum external EL quantum efficiency, ηc = maximum current efficiency, ηp = maximum power efficiency, CIE = Commission Internationale de l'Éclairage color coordinates measured at 10 mA cm−2, PPF = 2,8-bis(diphenylphosphoryl)dibenzofuran.
5. 5. Experimental Section 5. 5. 1. General Methods
Quantum chemical calculation for MFAc-AzB was performed using the Gaussian 09 program package. Geometries in the ground state were optimized using the B3LYP functional with the 6-31G(d) basis set. Low-lying excited singlet and triplet states were computed using the optimized structures with time-dependent density functional theory (TD-DFT) at the same level. NMR spectra were recorded on a Bruker Avance III 500 or Avance III 400 spectrometer.
Chemical shifts of 1H and 13C NMR signals were quoted to tetramethylsilane (δ = 0.00) and CDCl3 (δ = 77.0) as internal standards, respectively. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra were collected on an Autoflex III spectrometer (Bruker Daltonics) using dithranol as the matrix. Organic thin films for photophysical measurements were vacuum-deposited under high vacuum (~ 7 × 10−5 Pa) onto quartz glass and Si (100) substrates, which were pre-cleaned by acetone and isopropanol. UV–vis absorption spectra and photoluminescence (PL) spectra were measured with a Jasco V-670Y spectrometer and a Jasco FP-8600Y spectrophotometer, respectively, in degassed spectral grade solvents.
The photoluminescence quantum yields (PL) were determined using a Jasco ILF-835Y and calibrated integrating sphere system. The transient PL properties of doped films were evaluated using a Hamamatsu Photonics C9300 Streak camera with an N2 gas laser (λ = 337 nm, pulse width = 500 ps, repetition rate = 20 Hz) under vacuum (< 4 × 10−1 Pa). The HOMO energy level of the sample was determined using a Riken-Keiki AC-2 ultraviolet photoelectron spectrometer. The LUMO energy level was estimated by subtracting the optical energy gap (Eg) from the measured HOMO energy; the Eg value was determined from the onset position of the PL spectrum of a neat film.
87 5. 5. 2. Preparation of Materials
All regents and anhydrous solvents were purchased from Sigma-Aldrich, Tokyo Chemical Industry (TCI), or Wako Pure Chemical Industries, and were used as received unless otherwise noted. A host material, 2,8-bis(diphenylphosphine oxide)dibenzofuran (PPF) was prepared according to the literature procedure,[16] and then further purified by vacuum sublimation twice. Other OLED materials were purchased from Luminescence Technology Corporation. The detailed synthetic routes for MFAc-AzB are outlined in Scheme 5-1.
5. 5. 3. Synthesis
Scheme 5-1. Synthetic routes of MFAc-AzB.
MFAc-AzB was synthesized using Buchwald–Hartwing amination of a bromo-phenazoborin precursor (3) with spiroacridan (6) by employing Pd2(dba)3/P(t-Bu)3HBF4
catalytic system in 77% yield without affecting the boron center. The final product was purified by column chromatography, followed by temperature-gradient vacuum sublimation. Therefore, the chemical and thermal stability of MFAc-AzB are high enough for OLED applications because of effective steric protection of the 2,4,6-triisopropylphenyl group on the boron center.
88
Synthesis of 2,4-dibromo-N-(2-bromophenyl)aniline (1): A solution of 2,4-dibromoaniline (25.1 g, 100 mmol), sodium tert-butoxide (14.4 g, 150 mmol) in toluene (250 mL) was stirred for 30 min under nitrogen atmosphere at room temperature. Then, 1-bromo-2-iodobenzene (28.3 g, 100 mmol), Pd2(dba)3 (0.46 g, 0.5 mmol), and 1,1'-bis(diphenylphosphino)ferrocene (dppf, 0.54 g, 1.0 mmol) were added to the solution. The mixture was refluxed for 6 h. After cooling to room temperature, the precipitate was filtered through a Celite pad with chloroform, and the filtrate was concentrated under reduced pressure. The product was purified by column chromatography on silica gel (hexane/CH2Cl2 = 9:1, v/v), and recrystallized from hexane to give 1 as a white solid (yield = 31.3 g, 77%). 1H NMR (500 MHz, CDCl3): 7.71 (d, J = 2.2 Hz, 1H), 7.59 (dd, J = 8.0 Hz, 1.2 Hz, 1H), 7.31 (dd, J = 8.7 Hz, 2.2 Hz, 1H), 7.27-7.22 (m, 2H), 7.13 (dd, J = 8.7 Hz, 2.2 Hz, 1H), 6.90-6.86 (m, 1H), 6.38 (br, 1H); MS (MALDI-TOF) m/z: 402.98 [M]+, calcd 402.82.
Synthesis of 2,4-dibromo-N-(2-bromophenyl)-N-phenylaniline (2): A mixture of 1 (30.8 g, 75.8 mmol), iodobenzene (77.3 g, 379 mmol), CuI (14.4 g, 75.8 mmol), and K2CO3 (21.0 g, 151.6 mmol) was stirred for 72 h at 190 °C under nitrogen atmosphere. After cooling to room temperature, the precipitate was filtered through a Celite pad with toluene, and the filtrate was concentrated under reduced pressure. The product was purified by column chromatography on silica gel (hexane/CH2Cl2 = 9:1, v/v) to give 2 as a white solid (yield = 28.5 g, 78%). 1H NMR (500 MHz, CDCl3): 7.75 (d, J = 2.3 Hz, 1H), 7.63 (dd, J = 8.0 Hz, 1.5 Hz, 1H), 7.36 (dd, J = 8.6 Hz, 2.3 Hz, 1H), 7.28-7.20 (m, 3H), 7.09 (dd, J = 8.0 Hz, 1.6 Hz, 1H), 7.06 (td, J = 7.4 Hz, 1.60 Hz, 1H), 6.99 (dd, J = 7.4 Hz, 1.1 Hz, 1H), 6.95 (d, J = 8.6 Hz, 1H), 6.72 (dd, J = 7.8 Hz, 1.1 Hz, 2H); MS (MALDI-TOF) m/z: 479.07 [M]+, calcd 478.85.
Synthesis of 2-bromo-5-phenyl-10-(2,4,6-triisopropylphenyl)-5,10-dihydrodibenzo [b,e][1,4]azaborine (3): To a solution of 2 (28.0 g, 58.0 mmol) in dry THF (290 mL) was added dropwise n-butyllithium (1.6 M, 72.5 mL, 31.8 mmol) at −78 °C under nitrogen atmosphere. The mixture was stirred for 30 min at that temperature. Then, a solution of dimethyl (2,4,6-triisopropylphenyl)boronate (16.0 g, 58.0 mmol) in dry THF (30 mL) was added dropwise to the solution at −78 °C. The reaction mixture was allowed to react for 1 h at −78 °C, and then further reacted for 2 h at room temperature. After that, the mixture was reflux for 1 h.
After heating up to room temperature, the reaction was quenched by addition of a large amount of water. The product was extracted with chloroform and dried over anhydrous magnesium
89
sulfate. After filtration and evaporation, the crude product was purified by column chromatography on silica gel (hexane/CH2Cl2 = 9:1, v/v), and then recrystallized from acetonitrile to afford 3 as a white solid (yield = 21.4 g, 69%). 1H NMR (500 MHz, CDCl3): 8.02 (d, J = 2.5 Hz, 1H), 7.90 (dd, J = 7.6 Hz, 1.5 Hz, 1H), 7.73 (t, J = 7.2 Hz, 2H), 7.65 (tt, J
= 7.4 Hz, 1.1 Hz, 1H), 7.53-7.47 (m, 2H), 7.44 (dd, J = 8.4 Hz, 1.0 Hz, 2H), 7.12-7.09 (m, 3H), 6.80 (d, J = 8.7 Hz, 1H), 6.72 (d, J = 9.2 Hz, 1H), 3.02 (sep., J = 6.9 Hz, 1H), 2.42 (sep., J = 6.8 Hz, 2H), 1.39 (d, J = 6.9 Hz, 6H), 1.05 (d, J = 6.8 Hz, 6H), 1.02 (d, J = 6.8 Hz, 6H); MS (MALDI-TOF) m/z: 535.46 [M]+, calcd 535.20.
Synthesis of methyl-N-(p-tolyl)aniline (4): A solution of 2-bromo-4-methylaniline (48.4 g, 260 mmol), sodium tert-butoxide (37.5 g, 390 mmol) in dry toluene (520 mL) was stirred for 30 min under nitrogen atmosphere at room temperature. Then, 1-iodo-4-methylbenzene (64.2 g, 286 mmol), Pd(OAc)2 (2.38 g, 2.6 mmol), and 1,1'-bis(diphenylphosphino)ferrocene (dppf, 1.44 g, 2.6 mmol) were added to the solution. The mixture was refluxed for 8 h. After cooling to room temperature, the precipitate was filtered through a Celite pad with chloroform, and the filtrate was concentrated under reduced pressure.
The product was purified by column chromatography on silica gel (hexane/CH2Cl2 = 9:1, v/v) to give 4 as a white solid (yield = 54.8 g, 74%). 1H NMR (500 MHz, CDCl3): 7.56 (d, J = 8.3 Hz, 1H), 7.34 (d, J = 1.4 Hz, 1H), 7.10 (d, J = 8.0 Hz, 2H), 7.07 (d, J = 8.3 Hz, 1H), 7.00 (d, J
= 8.4 Hz 2H), 6.96-6.91 (m, 1H), 2.32 (s, 3H), 2.25 (s, 3H); MS (MALDI-TOF) m/z: 275.71 [M]+, calcd 275.03.
Synthesis of 9-(5-methyl-2-(p-tolylamino)phenyl)-9H-fluoren-9-ol (5): To a solution of 4 (21.0 g, 76.0 mmol) in dry THF (300 mL) was added dropwise n-butyllithium (1.6 M, 97.4 ml, 156 mmol) at −78 °C under nitrogen atmosphere. The mixture was stirred for 1 min at room temperature. After heating up to 0 °C, 9H-fluorenone (15.1 g, 83.6 mmol) was added to the mixture, and the mixture was further stirred for 6 h at room temperature. The reaction was quenched by addition of a large amount of water. The product was extracted with chloroform and dried over anhydrous magnesium sulfate. After filtration and evaporation, the product was used in the next reaction without further purification.
Synthesis 2,7-dimethyl-10H-spiro[acridine-9,9'-fluorene] (6): To a solution of crude 5 in dichloromethane (300 mL) was added methanesulfonic acid (14.6 g, 152 mmol) at room temperature. The mixture was refluxed for 1 h under air. After cooling to room temperature,
90
the mixture was concentrated under reduced pressure. The residue was purified by column chromatography on silica gel (hexane/CH2Cl2 = 4:1, v/v) to give 6 (yield = 12.2 g, 45%). 1H NMR (400 MHz, CDCl3): 7.78 (br, 1H), 7.37-7.18 (m, 14H); MS (MALDI-TOF) m/z: 358.98 [M]+ calcd 359.17; found, 358.98.
Synthesis of 2,7-dimethyl-10-(5-phenyl-10-(2,4,6-triisopropylphenyl)-5,10-dihydrodibenzo[b,e][1,4]azaborin-2-yl)-10H-spiro[acridine-9,9'-fluorene] (MFAc-AzB):
A solution of 6 (1.58 g, 4.4 mmol), sodium tert-butoxide (0.77 g, 8.0 mmol) in dry toluene (40 mL) was stirred for 30 min under nitrogen atmosphere at room temperature. To the solution were added 3 (2.15 g, 4.0 mmol), Pd2(dba)3 (0.073 g, 0.08 mmol), and P(t-Bu)3HBF4 (0.093 g, 0.32 mmol). The mixture was refluxed for 14 h. After cooling to room temperature, the precipitate was filtered through a Celite pad with chloroform, and the filtrate was concentrated under reduced pressure. The product was purified by column chromatography on silica gel (hexane/CH2Cl2 = 7:3, v/v), and recrystallized from acetonitrile to give MFAc-AzB as a light yellow solid (yield = 2.5 g, 77%). 1H NMR (500 MHz, CDCl3): 8.06 (d, J = 2.0 Hz, 1H), 7.97 (dd, J = 7.5 Hz, 1.2 Hz, 1H), 7.79-7.75 (m, 4H), 7.69 (t, J = 7.5 Hz, 1H), 7.59 (d, J = 7.3 Hz, 2H), 7.56-7.50 (m, 2H), 7.41 (d, J = 7.6 Hz, 2H), 7.34 (t, J = 7.3 Hz, 2H), 7.20-7.11 (m, 4H), 7.06 (s, 2H), 6.90 (d, J = 8.7 Hz, 1H), 6.61 (d, J = 8.4 Hz, 2H), 6.26 (d, J = 8.4 Hz, 2H), 6.13 (s, 2H), 2.97 (sep., J = 6.8 Hz, 1H), 2.57 (sep., J = 6.7 Hz, 2H), 1.90 (s, 6H), 1.35 (d, J = 6.9 Hz, 6H), 1.06 (d, J = 6.8 Hz, 6H), 1.03 (d, J = 6.8 Hz, 6H); 13C NMR (125 MHz, CDCl3, δ):
156.05, 150.41, 147.90, 146.31, 145.57, 141.64, 139.93, 139.66, 137.74, 134.89, 132.89, 132.52, 131.00, 130.41, 129.12, 129.04, 128.29, 127.83, 127.42, 127.34, 125.80, 124.77, 120.07, 119.92, 119.72, 116.90, 114.60, 56.97, 35.26, 34.24, 24.48, 24.44, 24.19, 20.38; HRMS (FAB) m/z: 814.44584 [M]+, calcd 814.44583.
5. 5. 4. OLED Fabrication and Measurements
Indium tin oxide (ITO)-coated glass substrates were cleaned with detergent, deionized water, acetone, and isopropanol, and then treated with UV-ozone treatment for 15 min, before being loaded into a vacuum evaporation system. The organic layers were thermally evaporated on the substrates under vacuum (< 6 × 10−5 Pa) with an evaporation rate of < 0.3 nm s−1. A cathode aluminum layer was then deposited through a shadow mask. The layer thickness and deposition rate were monitored in situ during deposition by an oscillating quartz thickness
91
monitor. OLED device properties were measured using a Keithley source meter 2400 and a Konica Minolita CS-2000
5. 6. Conclusion
A new phenazaborin-based luminophore, MFAc-AzB, have been presented for OLED application. A highly efficient blue TADF material exhibiting a high PL of 99% has been successfully developed by combining phenazaborin acceptor and spiroacridan donor units. An OLED employing MFAc-AzB as a blue TADF emitter has achieved a notably high maximum
ext of 18.2%. The molecular design featuring the phenazaborin core can provide a promising way for further developing sophisticated high-efficiency blue TADF molecules for OLED applications.
92 References
[1] H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, Nature 2012, 492, 234.
[2] a) S. Y. Lee, T. Yasuda, H. Nomura, C. Adachi, Appl. Phys. Lett. 2012, 101, 093306; b) S. Hirata, Y. Sakai, K. Masui, H. Tanaka, S. Y. Lee, H. Nomura, N. Nakamura, M.
Yasumatsu, H. Nakanotani, Q. Zhang, K. Shizu, H. Miyazaki, C. Adachi, Nat. Mater.
2015, 14, 330.
[3] a) S. Y. Lee, T. Yasuda, Y. S. Yang, Q. Zhang, C. Adachi, Angew. Chem. Int. Ed. 2014, 126, 6520; b) S. Y. Lee, T. Yasuda, I. S. Park, C. Adachi, Dalton Trans. 2015, 44, 8356.
[4] a) Q. Zhang, B. Li, S. Huang, H. Nomura, H. Tanaka, C. Adachi, Nat. Photon. 2014, 8, 326; b) Q. Zhang, J. Li, K. Shizu, S. Huang, S. Hirata, H. Miyazaki, C. Adachi, J. Am.
Chem. Soc. 2012, 134, 14706.
[5] K. Nasu, T. Nakagawa, H. Nomura, C.-J. Lin, C.-H. Cheng, M.-R. Tseng, T. Yasuda, C. Adachi, Chem. Commun. 2013, 49, 10385.
[6] a) M. Numata, T. Yasuda, C. Adachi, Chem. Commun. 2015, 51, 9443; b) W. Sun, J. Y.
Baek, K.-H. Kim, C.-K. Moon, J.-H. Lee, S.-K. Kwon, Y.-H. Kim, J.-J. Kim, Chem.
Mater. 2015, 27, 6675; c) M. Kim, S. K. Jeon, S.-H. Hwang, J. Y. Lee, Adv. Mater.
2015, 27, 2515.
[7] a) T. Agou, J. Kobayashi, T. Kawashima, Chem. Commun. 2007, 3204; b) T. Ago, J.
Kobayashi, T. Kawashima, Org. Lett. 2006, 8, 2241; c) T. Agou, M. Sekine, J.
Kobayashi, T. Kawashima, Chem. Commun. 2009, 1894; d) T. Agou, T. Kojima, J.
Kobayashi, T. Kawashima, Org. Lett. 2009, 11, 3534; e) A. Wakamiya, K. Mori, S.
Yamaguchi, Angew. Chem. Int. Ed. 2007, 46, 4273; f) C.-H. Zhao, A. Wakamiya, Y.
Inukai, S. Yamaguchi, J. Am. Chem. Soc. 2006, 128, 15934; g) T. Noda, Y. Shirota, J.
Am. Chem. Soc. 1998, 120, 9714; b) H. Doi, M. Kinoshita, K. Okumoto, Y. Shirota, Chem. Mater. 2003, 15, 1080; h) N. Matsumi, K. Naka, Y. Chujo, J. Am. Chem. Soc.
1998, 120, 5112; i) C. D. Entwistle, T. B. Marder, Chem. Mater. 2004, 16, 4574; j) W.
L. Jia, X. D. Feng, D. R. Bai, Z. H. Lu, S. Wang, G. Vamvounis, Chem. Mater. 2005, 17, 164.
[8] P. A. Vecchi, A. B. Padmaperuma, H. Qiao, L. S. Sapochak, P. E. Burrows, Org. Lett.
2006, 8, 4211.
[9] A. Qin, B. Z. Tang, Aggregation-Induced Emission: Fundamentals, Wiley, United Kingdom, 2013.
93
[10] R. J. Holmes, S. R. Forrest, Y.-J. Tung, R. C. Kwong, J. J. Brown, S. Garon, M. E.
Thompson, Appl. Phys. Lett. 2003, 82, 2422.
94